Uranium oxide manufacturing plants implementing a dry method of manufacturing uranium oxide from gaseous uranium hexafluoride UF6 coming from a plant for enriching uranium used as a nuclear fuel with radioactive isotopes.
Such plants implementing the method of manufacturing uranium oxide from uranium hexafluoride, called dry conversion method, comprise, in an industrial embodiment which is currently the most satisfactory and most widespread embodiment, a reactor having an overall vertical arrangement into which uranium hexafluoride UF6, steam and dilution nitrogen are introduced, at a temperature of 200° C. to 300° C., and uranium hexafluoride UF6 is converted into uranium oxyfluoride UO2F2 according to the hydrolysis reaction:UF6+2H2O→UO2F2+4HF.
The conversion of uranium hexafluoride into uranium oxyfluoride by hydrolysis produces hydrofluoric acid HF in gaseous form and uranium oxyfluoride UO2F2 in powder form.
The uranium oxyfluoride UO2F2 falls and is deposited in the bottom of the reactor where it is taken up by a feed screw transferring it to the input of a rotary furnace in which the uranium oxyfluoride UO2F2 is converted into uranium oxides UO2+x, that is to say into uranium oxides UO2 with a superstoichiometric composition, the O/U ratio of the oxide varying between 2.03 and 2.66, according to the conditions for carrying out the conversion in the rotary furnace.
Hydrofluoric acid HF is evacuated through the upper part of the uranium hexafluoride conversion reactor, through filtration units which retain the uranium oxyfluoride powder UO2F2 conveyed by the gaseous hydrofluoric acid in a mixture with gases such as N2 and H2O sucked through the upper part of the conversion reactor.
Generally, at least two filtration units or blocks are used, arranged in the upper part of the reactor through which the gases containing hydrofluoric acid are evacuated.
Each of the filtration units comprises a gas evacuation manifold, or head, communicating via a pipe, outside the conversion reactor, with a plant enabling the gases to be routed toward a safety filter located outside the reactor downstream of the gas routing plant, arranged in a hot box, and toward a unit for recovering hydrofluoric acid.
Inside the conversion reactor, the filtration units comprise a set of filters, or filter cartridges, each one comprising a filtering wall of overall cylindrical shape placed in the upper part of the conversion reactor with its axis vertical.
The filters, or filtering cartridges, of each of the filtration units rest, and are fastened, via an upper flange, onto a plate of the filtration unit arranged horizontally and separating the manifold or head of the filtration unit from the internal volume of the reactor in which the chemical reaction forming uranium oxyfluoride takes place.
The gases, loaded to a greater or lesser degree with powdered uranium oxyfluoride, come into contact with the outer surface of the filtering cartridges and, after separation of the powdered uranium oxyfluoride, cross the wall of the filtering cartridge thereby entering the head of the filtration unit. The powdered uranium oxyfluoride stopped by the outer wall of the filtration unit is able to fall back by gravity into the bottom of the reactor where it is taken up by the feed screw for conveying the powder.
The tubular filtration walls of the filtering cartridges, which generally consist of compressed and sintered particles or fibers, have pores to allow the passage of gases containing hydrofluoric acid, the size of which is less than the size of the uranium oxyfluoride particles, so as to stop all the solid particles which may be conveyed by the gases.
Some of the uranium oxyfluoride particles in powder form are deposited on the outer surface of the tubular wall of the cartridge, such that the filters gradually clog up during plant operation. Passage of the mixed hydrofluoric acid and gases through the wall of the filters occurs with an increasing pressure drop which is incompatible with continuous operation of the plant.
It is therefore necessary to declog the walls of the filtering cartridges periodically and sufficiently frequently.
This declogging is performed by injecting, inside the filtering cartridges, counter-currentwise with respect to the direction of flow of the gases containing the hydrofluoric acid, an inert declogging gas such as nitrogen.
The declogging nitrogen must be delivered into the filtering cartridges with an overpressure of about 2.5 bar and at a temperature of about 130° C., the temperature in the conversion reactor being about 300° C. and the temperature of the gases at the reactor outlet being about 200° C. to 300° C.
The declogging nitrogen which is contained in a storage tank is delivered to the filtration units, via the gas routing plant located above the conversion reactor. This gas routing plant is arranged inside a heated chamber, whose internal temperature is about 150° C.
Valves are arranged inside the heated chamber, in particular three-way valves, making it possible, on the one hand, for the gases containing the hydrofluoric acid to pass to the safety filter and the recovery plant, during the phases of normal operation of the filtration unit and, on the other hand, for the declogging nitrogen to pass in the opposite direction to the gas flow, during the declogging phases.
The valves withstand high temperatures and, because they operate at their upper temperature limit, their use and their maintenance are complex and expensive.
To allow continuous operation of the plant, declogging is carried out on one of the filtration units while the second filtration unit alone evacuates the gases containing the hydrofluoric acid produced in the reactor. As a result, during the declogging phases, the filtration unit remaining in operation must evacuate a gas flow which is roughly twice its usual operational flow. Furthermore, the flow of declogging nitrogen, which is delivered into the second filtration unit, to the inside of the conversion reactor, must be evacuated, which further increases the gas flow which must be evacuated by the filtration unit remaining in operation.
In one known embodiment of the filtration units of a uranium hexafluoride conversion reactor, each of the filtration units comprises eight filtering cartridges which are distributed in the plane of the horizontal plate for supporting the filters separating the head for recovering hydrofluoric acid from the internal volume of the reactor.
Declogging nitrogen is injected into the head through the pipe for evacuating gases containing hydrofluoric acid, in a region located in the central part of the head, that is to say, directly above the central part of the filter support plate. The result of this is that the jet of declogging nitrogen which is directed to the central part of the plate and to the filters carried by this central part performed preferential declogging of the filters located in the central part. These filters are perfectly declogged while the filters located on the periphery of the plate supporting the filtration unit are not sufficiently declogged. These filters become fouled, so that the pressure increases and operation of the conversion plant is disturbed, the gases passing in a preferential manner through the filters located in the central part, which are efficiently declogged.